Plumes of smoke are seen on Mount Etna during an eruption October 25, 2013. Etna's eruptions aren't infrequent, although the last major one occurred in 1992. (Photo by Salvatore Allegra/Associated Press)

Lava flows during an eruption of Mt. Etna, seen from the village of Viagrande, near the Sicilian town of Catania on October 26, 2013. (Photo by Carmelo Imbesi/Associated Press)

Mt. Etna spews lava during an eruption, seen from Acireale, near the Sicilian town of Catania on November 16, 2013. (Photo by Carmelo Imbesi/Associated Press)

Mt. Etna spews lava during an eruption, seen from Acireale, near the Sicilian town of Catania. (Photo by Carmelo Imbesi/Associated Press)

A photographer takes pictures of Mount Etna spewing lava as it erupts on November 17, 2013. (Photo by Antonio Parrinello/Reuters)

In this photo taken on Saturday, November 16, 2013, Mt. Etna, Europe's most active volcano, spews lava during an eruption as seen from Acireale, near the Sicilian town of Catania, Italy. (Photo by Carmelo Imbesi/AP Photo)

People watch Italy's Mt. Etna spewing lava as it erupts on the southern island of Sicily. (Photo by Antonio Parrinello/Reuters)

Mt. Etna spews lava as it erupts on the southern island of Sicily. (Photo by Antonio Parrinello/Reuters)

Mt. Etna spews lava during an eruption as seen from Acireale. (Photo by Carmelo Imbesi/Associated Press)